The present disclosure relates to methods and devices that permit dynamic stabilization of the bony elements of the skeleton. The devices permit adjustment and maintenance of the spatial relationship(s) between neighboring bones. Depending on the specifics of the design, the motion between skeletal segments may be limited or enhanced in one or more planes.
Spinal degeneration is an unavoidable consequence of aging. The disability produced by the aging spine has emerged as a major health problem in the industrialized world. Alterations in the anatomical alignment and physiologic motion that normally exists between adjacent spinal vertebrae can cause significant pain, deformity, weakness, and catastrophic neurological dysfunction. The traditional surgical treatment of spinal disease is decompression of the neural elements and complete immobilization of the involved bony spinal segments. Over time, an extensive array of surgical techniques and implantable devices has been formulated to accomplish this goal.
The growing experience with spinal fusion has shed light on the long-term consequences of vertebral immobilization. It is now accepted that fusion of a specific spinal level will increase the load on, and the rate of degeneration of, the spinal segments immediately above and below the fused level. As the number of spinal fusion operations have increased, so have the number of patients who require extension of their fusion to the adjacent, degenerating levels. The second procedure necessitates re-dissection through the prior, scarred operative field and carries significantly greater risk than the initial procedure while providing a reduced probability of pain relief. Further, extension of the fusion will increase the load on the motion segments that now lie at either end of the fusion construct and will accelerate the rate of degeneration at those levels. Thus, spinal fusion begets additional fusion surgery.
In view of the proceeding, there is a growing recognition that segmental spinal fusion and complete immobilization is an inadequate solution to abnormal spinal motion and vertebral mal-alignment. Correction of the abnormal movement and preservation of spinal mobility is a more intuitive and rational treatment plan.
The vast experience gained in the implantation of mobile prostheses in the hip, knee, shoulder, ankle, digits and other joints of the extremities has shown that the wear debris produced by the bearing surfaces and the loosening that occur at the bone-device interface are major causes of implant failure. The latter is at least partially caused by the former, since it's been shown that the particulate debris from the bearing surfaces promote bone re-absorption at the bone-device interface and significantly accelerates device loosening. In the long term, the degradation products of the implant materials may also produce negative biological effects at distant tissues within the implant recipient.
While ceramic and polymer implant components produce wear debris, these degradation products are usually deposited as insoluble particles around the implant thereby limiting the extent of potential toxicity. In contrast, metallic degradation products may be present as particulate and corrosion debris as well as free metals ions, composite complexes, inorganic metal salts/oxides, colloidal organo-metallic complexes and other molecules that may be transported to distant body sites. In fact, studies have revealed chronic elevations in serum and urine cobalt and chromium level after prosthetic joint replacement. Given the known toxicity of titanium, cobalt, chromium, nickel, vanadium, molybdenum and other metals used in the manufacture of orthopedic implants, the tissue distribution and biologic activity of their degradation products is of considerable concern. Host toxicity may be produced directly by the reactive metallic moieties as well as by their alterations of the immune system, metabolic function, and their potential ability to cause cancer. These issues are thoroughly discussed in the text “Implant Wear in Total Joint replacement” edited by Thomas Wright and Stuart Goodman and published by the American Academy of Orthopedic Surgeons in 2000. The text is hereby incorporated by reference in its entirety.
Unlike joints in the extremities, proper function of the spinal joints (i.e., inter-vertebral disc and facet joints) returns the attached bones to the neutral position after the force producing the motion has dissipated. That is, a force applied to the hip, knee or other joints of the extremities produces movement in the joint and a change in the position of the attached bones. After the force has dissipated, the bones remain in the new position until a second force is applied to them. In contrast, the visco-elastic properties of the spinal disc and facet joint capsule dampen the force of movement and return the vertebral bones to a neutral position after the force acting upon them has dissipated.
Prosthetic joint implants that attempt to imitate native spinal motion have usually employed springs, polyurethane, rubber and the like to recreate the visco-elastic properties of the spinal joints. When subjected to the millions of cycles of repetitive loading that is required of a spinal joint prosthesis, all implants to date have been plagued by excessive wear and degeneration secondary to the fairly modest wear characteristics of these elastic elements. Thus, in addition to the wear debris generated by the bearing surface(s), the elastic materials used to dampen spinal motion will produce a second source of degradation products. Given the number of joints in the spine and the extensive potential application of replacement technology in these joints, it is critical that the wear debris from the implanted prosthesis be minimized.